The present invention relates to a liquid-crystalline medium, to the use thereof for electro-optical purposes, and to displays containing this medium.
Liquid-crystals are used principally as dielectrics in display devices, since the optical properties of such substances can be modified by an applied voltage. Electro-optical devices based on liquid crystals are extremely well known to the person skilled in the art and can be based on various effects. Examples of such devices are cells having dynamic scattering, DAP (deformation of aligned phases) cells, guest/host cells, TN cells having a twisted nematic structure, STN (supertwisted nematic) cells, SBE (superbirefringence effect) cells and OMI (optical mode interference) cells. The commonest display devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
The liquid-crystal materials must have good chemical and thermal stability and good stability to electric fields and electromagnetic radiation. Furthermore, the liquid-crystal materials should have low viscosity and produce short addressing times, low threshold voltages and high contrast in the cells.
They should furthermore have a suitable mesophase, for example a nematic or cholesteric mesophase for the above-mentioned cells, at the usual operating temperatures, i.e. in the broadest possible range above and below room temperature. Since liquid crystals are generally used as mixtures of a plurality of components, it is important that the components are readily miscible with one another. Further properties, such as the electrical conductivity, the dielectric anisotropy and the optical anisotropy, have to satisfy various requirements depending on the cell type and area of application. For example, materials for cells having a twisted nematic structure should have positive dielectric anisotropy and low electrical conductivity.
For example, for matrix liquid-crystal displays with integrated non-linear elements for switching individual pixels (MLC displays), media having large positive dielectric anisotropy, broad nematic phases, relatively low birefringence, very high specific resistance, good UV and temperature stability and lower vapor pressure are desired.
Matrix liquid-crystal displays of this type are known. Non-linear elements which can be used for individual switching of the individual pixels are, for example, active elements (i.e. transistors). The term xe2x80x9cactive matrixxe2x80x9d is then used, where a distinction can be made between two types:
1. MOS (metal oxide semiconductor) or other diodes on a silicon wafer as substrate.
2. Thin-film transistors (TFTs) on a glass plate as substrate.
The use of single-crystal silicon as substrate material restricts the display size, since even modular assembly of various part-displays results in problems at the joins.
In the case of the more promising type 2, which is preferred, the electro-optical effect used is usually the TN effect. A distinction is made between two technologies: TFTs comprising compound semiconductors, such as, for example, CdSe, or TFTs based on polycrystalline or amorphous silicon. Intensive work is being carried out world-wide on the latter technology.
The TFT matrix is applied to the inside of one glass plate of the display, while the other glass plate carries the transparent counterelectrode on its inside. Compared with the size of the pixel electrode, the TFT is very small and has virtually no adverse effect on the image. This technology can also be extended to fully color-capable displays, in which a mosaic of red, green and blue filters is arranged in such a way that a filter element is opposite each switchable pixel.
The TFT displays usually operate as TN cells with crossed polarizers in transmission and are illuminated from the back.
The term MLC displays here covers any matrix display with integrated non-linear elements, i.e., besides the active matrix, also displays with passive elements, such as varistors or diodes (MIM=metal-insulator-metal).
MLC displays of this type are particularly suitable for TV applications (for example pocket TVs) or for high-information displays for computer applications (laptops) and in automobile or aircraft construction. Besides problems regarding the angle dependence of the contrast and the response times, difficulties also arise in MLC displays due to insufficiently high specific resistance of the liquid-crystal mixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIUMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September 1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p. 141 ff, Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Design of Thin Film Transistors for Matrix Addressing of Television Liquid Crystal Displays, p. 145 ff, Paris]. With decreasing resistance, the contrast of an MLC display deteriorates, and the problem of after-image elimination may occur. Since the specific resistance of the liquid-crystal mixture generally drops over the life of an MLC display owing to interaction with the interior surfaces of the display, a high (initial) resistance is very important in order to obtain acceptable service lives. In particular in the case of low-volt mixtures, it was hitherto impossible to achieve very high specific resistance values. It is furthermore important that the specific resistance exhibits the smallest possible increase with increasing temperature and after heating and/or UV exposure. The low-temperature properties of the mixtures from the prior art are also particularly disadvantageous. It is demanded that no crystallization and/or smectic phases occur, even at low temperatures, and the temperature dependence of the viscosity is as low as possible. The MLC displays from the prior art thus do not meet today""s requirements.
There thus continues to be a great demand for MLC displays having very high specific resistance at the same time as a large working-temperature range, short response times even at low temperatures and low threshold voltage which do not have these disadvantages, or only do so to a reduced extent.
In TN (Schadt-Helfrich) cells, media are desired which facilitate the following advantages in the cells:
extended nematic phase range (in particular down to low temperatures)
stable on storage, even at low temperatures
the ability to switch at extremely low temperatures (outdoor use, automobile, avionics)
increased resistance to UV radiation (longer service life).
The media available from the prior art do not allow these advantages to be achieved while simultaneously retaining the other parameters.
In the case of supertwisted (STN) cells, media are desired which enable greater multiplexability and/or lower threshold voltages and/or broader nematic phase ranges (in particular at low temperatures). To this end, a further widening of the available parameter latitude (clearing point, smectic-nematic transition or melting point, viscosity, dielectric parameters, elastic parameters) is urgently desired.
The invention has an object of providing media, in particular for MLC, TN or STN displays of this type, which do not have the above-mentioned disadvantages or only do so to a reduced extent, and preferably simultaneously have very high specific resistance values and low threshold voltages.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
It has now been found that these and other objects can be achieved if media according to the invention are used in displays.
The invention thus relates to a liquid-crystalline medium based on a mixture of polar compounds of positive dielectric anisotropy, characterized in that it comprises one or more compounds of the formula I 
in which
R1 is a halogenated or unsubstituted alkyl or alkoxy radical having from 1 to 15 carbon atoms, where, in addition, one or more CH2 groups in these radicals may each, independently of one another, be replaced by xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94 or xe2x80x94Oxe2x80x94COxe2x80x94 in such a way that O atoms are not linked directly to one another,
X is F, Cl, CN, SF5, a halogenated alkyl radical, a halogenated alkenyl radical, a halogenated alkoxy radical or a halogenated alkenyloxy radical having up to 6 carbon atoms,
L1 and L2 are each, independently of one another, H or F.
The compounds of the formula I have a broad range of applications. Depending on the choice of substituents, these compounds can serve as base materials of which liquid-crystalline media are predominantly composed; however, it is also possible to add compounds of the formula I liquid-crystalline base materials from other classes of compound in order, for example, to modify the dielectric and/or optical anisotropy of a dielectric of this type and/or in order to optimize its threshold voltage and/or its viscosity.
In the pure state, the compounds of the formula I are colorless and form liquid-crystalline mesophases in a temperature range which is favorably located for electro-optical use. They are stable chemically, thermally and to light.
If R1 in the formula I is an alkyl radical and/or an alkoxy radical, this may be straight-chain or branched. It is preferably straight-chain, has 2, 3, 4, 5, 6 or 7 carbon atoms and accordingly is preferably ethyl, propyl, butyl, pentyl, hexyl, heptyl, ethoxy, propoxy, butoxy, pentoxy, hexyloxy or heptyloxy, furthermore methyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, methoxy, octyloxy, nonyloxy, decyloxy, undecyloxy, dodecyloxy, tridecyloxy or tetradecyloxy.
Oxaalkyl is preferably straight-chain 2-oxapropyl (=methoxymethyl), 2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-, 3- or 4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl, 2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5- , 6-, 7- or 8-oxanonyl, or 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl.
If R1 is an alkyl radical in which one CH2 group has been replaced by xe2x80x94CHxe2x95x90CHxe2x80x94, this may be straight-chain or branched. It is preferably straight-chain and has 2 to 10 carbon atoms. Accordingly, it is in particular vinyl, prop-1- or prop-2-enyl, but-1-, -2- or -3-enyl, pent-1-, -2-, -3- or -4-enyl, hex-1-, -2-, -3-, -4- or -5-enyl, hept-1-, -2-, -3-, -4-, -5- or -6-enyl, oct-1-, -2-, -3-, -4-, -5-, -6- or -7-enyl, non-1-, -2-, -3-, -4-, -5-, -6-, -7- or -8-enyl, or dec-1-, -2-, -3-, -4-, -5-, -6-, -7-, -8- or -9-enyl.
If R1 is an alkyl radical in which one CH2 group has been replaced by xe2x80x94Oxe2x80x94 and one has been replaced by xe2x80x94COxe2x80x94, these are preferably adjacent. These thus contain an acyloxy group xe2x80x94COxe2x80x94Oxe2x80x94 or an oxycarbonyl group xe2x80x94Oxe2x80x94CO. These are preferably straight-chain and have 2 to 6 carbon atoms. Accordingly, they are in particular acetoxy, propionyloxy, butyryloxy, pentanoyloxy, hexanoyloxy, acetoxymethyl, propionyloxymethyl, butyryloxymethyl, pentanoyloxymethyl, 2-acetoxyethyl, 2-propionyloxyethyl, 2-butyryloxyethyl, 3-acetoxypropyl, 3-propionyloxypropyl, 4-acetoxybutyl, methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl, butoxycarbonyl, pentoxycarbonyl, methoxycarbonylmethyl, ethoxycarbonylmethyl, propoxycarbonylmethyl, butoxycarbonylmethyl, 2-(methoxycarbonyl)ethyl, 2-(ethoxycarbonyl)ethyl, 2-(propoxycarbonyl)ethyl, 3-(methoxycarbonyl)propyl, 3-(ethoxycarbonyl)propyl or 4-(methoxycarbonyl)butyl.
If R1 is an alkyl radical in which one CH2 group has been replaced by unsubstituted or substituted xe2x80x94CHxe2x95x90CHxe2x80x94 and an adjacent CH2 group has been replaced by CO or COxe2x80x94O or Oxe2x80x94CO, this may be straight-chain or branched. It is preferably straight-chain and has 4 to 12 carbon atoms. Accordingly, it is in particular acryloyloxymethyl, 2-acryloyloxyethyl, 3-acryloyloxypropyl, 4-acryloyloxybutyl, 5-acryloyloxypentyl, 6-acryloyloxyhexyl, 7-acryl-oyloxyheptyl, 8-acryloyloxyoctyl, 9-acryloyloxynonyl, 10-acryloyloxydecyl, methacryloyloxymethyl, 2-methacryloyloxyethyl, 3-methacryloyloxypropyl, 4-meth-acryloyloxybutyl, 5-methacryloyloxypentyl, 6-methacryloyloxyhexyl, 7-methacryloyloxyheptyl, 8-methacryloyloxyoctyl or 9-methacryloyloxynonyl.
If R1 is an alkyl or alkenyl radical which is monosubstituted by CN or CF3, this radical is preferably straight-chain. The substitution by CN or CF3 is in any desired position.
If R1 is an alkyl or alkenyl radical which is at least monosubstituted by halogen, this radical is preferably straight-chain, and halogen is preferably F or Cl. In the case of polysubstitution, halogen is preferably F. The resultant radicals also include perfluorinated radicals. In the case of monosubstitution, the fluorine or chlorine substituent may be in any desired position, but is preferably in the co-position.
Compounds containing branched wing groups R1 may occasionally be of importance owing to better solubility in the conventional liquid-crystalline base materials, but in particular as chiral dopants if they are optically active. Smectic compounds of this type are suitable as components of ferroelectric materials.
Branched groups of this type generally contain not more than one chain branch. Preferred branched radicals R1 are isopropyl, 2-butyl (=1-methylpropyl), isobutyl (=2-methylpropyl), 2-methylbutyl, isopentyl (=3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl, 2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy, 3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexyloxy, 1-methylhexyloxy and 1-methylheptyloxy.
If R1 is an alkyl radical in which two or more CH2 groups have been replaced by xe2x80x94Oxe2x80x94 and/or xe2x80x94COxe2x80x94Oxe2x80x94, this may be straight-chain or branched. It is preferably branched and has from 3 to 12 carbon atoms. Accordingly, it is in particular biscarboxymethyl, 2,2-biscarboxyethyl, 3,3-biscarboxypropyl, 4,4-biscarboxybutyl, 5,5-biscarboxypentyl, 6,6-biscarboxyhexyl, 7,7-biscarboxyheptyl, 8,8-biscarboxyoctyl, 9,9-biscarboxynonyl, 10,10-biscarboxydecyl, bis(methoxycarbonyl)methyl, 2,2-bis(methoxycarbonyl)ethyl, 3,3-bis(methoxycarbonyl)propyl, 4,4-bis(methoxycarbonyl)butyl, 5,5-bis(methoxycarbonyl)pentyl, 6,6-bis(methoxycarbonyl)hexyl, 7,7-bis(methoxycarbonyl)heptyl, 8,8-bis(methoxycarbonyl)octyl, bis(ethoxycarbonyl)methyl, 2,2-bis(ethoxycarbonyl)ethyl, 3,3-bis(ethoxycarbonyl)propyl, 4,4-bis(ethoxycarbonyl)butyl or 5,5-bis(ethoxycarbonyl)hexyl.
The compounds of the formula I are prepared by methods known per se, as described in the literature (for example in the standard works, such as Houben-Weyl, Methoden der organischen Chemie [Methods of Organic Chemistry], Georg-Thieme-Verlag, Stuttgart), to be precise under reaction conditions which are known and suitable for the said reactions. Use can also be made here of variants which are known per se, but are not mentioned here in greater detail. The compounds of the formula I can be prepared, for example, as described in DE 4006921, WO 01/64667 and DE 10105314.
The invention also relates to electro-optical displays (in particular STN or MLC displays having two plane-parallel outer plates, which, together with a frame, form a cell, integrated non-linear elements for switching individual pixels on the outer plates, and a nematic liquid-crystal mixture of positive dielectric anisotropy and high specific resistance which is located in the cell) which contain media of this type, and to the use of these media for electro-optical purposes.
The liquid-crystal mixtures according to the invention enable a significant widening of the available parameter latitude.
The achievable combinations of clearing point, viscosity at low temperature, thermal and UV stability and dielectric anisotropy are far superior to previous materials from the prior art.
The requirement for a high clearing point, a nematic phase at low temperature and a high xcex94xcex5 has hitherto only been achieved to an inadequate extent. Although mixtures such as, for example, ZLI-3119 have a comparable clearing point and comparably favorable viscosities, they have, however, a xcex94xcex5 of only +3. Other mixture systems have comparable viscosities and xcex94xcex5 values, but only have clearing points in the region of 60xc2x0 C.
The liquid-crystal mixtures according to the invention, for example while retaining the nematic phase down to xe2x88x9220xc2x0 C. and preferably down to xe2x88x9230xc2x0 C., particularly preferably down to xe2x88x9240xc2x0 C., preferably enable a clearing point above 60xc2x0 C., more preferably above 65xc2x0 C., particularly preferably above 70xc2x0 C., simultaneously dielectric anisotropy values xcex94xcex5 preferably of xe2x89xa76, more preferably xe2x89xa78, and a high value for the specific resistance to be achieved, enabling excellent STN and MLC displays to be obtained. In particular, the mixtures are characterized by low operating voltages. The TN thresholds are preferably below 2.0 V, more preferably below 1.7 V, particularly preferably  less than 1.3 V.
It goes without saying that, through a suitable choice of the components of the mixtures according to the invention, it is also possible for higher clearing points (for example above 110xc2x0 C.) to be achieved at a higher threshold voltage or lower clearing points to be achieved at lower threshold voltages with retention of the other advantageous properties. At viscosities correspondingly increased only slightly, it is likewise possible to obtain mixtures having greater xcex94xcex5 and thus lower thresholds. The MLC displays according to the invention preferably operate at the first Gooch and Tarry transmission minimum [C. H. Gooch and H. A. Tarry, Electron. Lett. 10, 2-4, 1974; C. H. Gooch and H. A. Tarry, Appl. Phys., Vol. 8, 1575-1584, 1975] are used, where, besides particularly favourable electro-optical properties, such as, for example, high steepness of the characteristic line and low angle dependence of the contrast (German Patent 30 22 818), a lower dielectric anisotropy is sufficient at the same threshold voltage as in an analogous display at the second minimum. This enables significantly higher specific resistances to be achieved using the mixtures according to the invention at the first minimum than in the case of mixtures comprising cyano compounds. Through a suitable choice of the individual components and their proportions by weight, the person skilled in the art is able to set the birefringence necessary for a pre-specified layer thickness of the MLC display using simple routine methods.
The flow viscosity xcexd20 at 20xc2x0 C. is preferably  less than 60 mm2xc2x7sxe2x88x921, particularly preferably  less than 50 mm2xc2x7sxe2x88x921. The rotational viscosity xcex31 at 20xc2x0 C. of the mixtures according to the invention is preferably  less than 160 mPaxc2x7s, particularly preferably  less than 150 mPaxc2x7s. The nematic phase range is preferably at least 90xc2x0, in particular at least 100xc2x0. This range preferably extends at least from xe2x88x9220xc2x0 to +80xc2x0 C.
A short response time is desired in liquid-crystal displays. This applies in particular to displays which are capable of video reproduction. For displays of this type, response times (total: ton+toff) of at most 25 ms are required. The upper limit of the response time is determined by the image refresh frequency. Besides the rotational viscosity xcex31, the tilt angle likewise affects the response time. In particular, mixtures comprising xe2x89xa720% of the compounds of the formula I exhibit a tilt angle of  greater than 2.5, preferably  greater than 3.0, compared with the commercial product ZLI-4792 from Merck KGaA.
Measurements of the voltage holding ratio (HR) [S. Matsumoto et al., Liquid Crystals 5, 1320 (1989); K. Niwa et al., Proc. SID Conference, San Francisco, June 1984, p. 304 (1984); G. Weber et al., Liquid Crystals 5, 1381 (1989)] have shown that mixtures according to the invention comprising compounds of the formula I exhibit a significantly smaller decrease in the HR with increasing temperature than, for example, analogous mixtures comprising cyanophenylcyclohexanes of the formula 
or esters of the formula 
instead of the compounds of the formula I.
The UV stability of the mixtures according to the invention is also considerably better, i.e. they exhibit a significantly smaller decrease in the HR on exposure to UV.
Particularly preferred compounds of the formula I are compounds of the formulae I-1 to I-9: 
in which R1 is as defined in the formula I.
Of these preferred compounds, particular preference is given to those of the formulae I-1, I-2, I-3 and I-4, in particular those of the formulae I-1 and 1-2.
Preferred embodiments are indicated below:
the medium comprises one, two or more compounds of the formulae I-1 to 1-9;
the medium additionally comprises one or more compounds selected from the group consisting of the general formulae IL to VI: 
in which the individual radicals have the following meanings:
R0 is n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having up to 9 carbon atoms,
X0 is F, Cl, halogenated alkyl, halogenated alkenyl, halogenated alkenyloxy or halogenated alkoxy having up to 6 carbon atoms,
Z0 is xe2x80x94C2F4xe2x80x94, xe2x80x94CFxe2x95x90CFxe2x80x94, xe2x80x94C2H4xe2x80x94, xe2x80x94(CH2)4xe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94CF2Oxe2x80x94 or xe2x80x94OCF2xe2x80x94,
Y1 to Y4 are each, independently of one another, H or F,
r is 0 or 1.
The compound of the formula IV is preferably 
the medium additionally comprises one or more compounds selected from the group consisting of the general formulae VII to XIII: 
in which R0, X0 and Y1-4 are each, independently of one another, as defined herein. X0 is preferably F, Cl, CF3, OCF3 or OCHF2. R0 is preferably alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having up to 6 carbon atoms.
The medium additionally comprises one or more compounds of the formulae E-a to E-d 
in which R0 is as defined herein;
the proportion of the compounds of the formulae E-a to E-d is preferably 10-30% by weight, in particular 15-25% by weight;
the proportion of compounds of the formulae I to VI together in the mixture as a whole is at least 50% by weight;
the proportion of compounds of the formula I in the mixture as a whole is from 0.5 to 40% by weight, particularly preferably from 1 to 30% by weight;
the proportion of compounds of the formulae II to VI in the mixture as a whole is from 30 to 80% by weight; 
the medium comprises compounds of the formulae II, III, IV, V and/or VI;
R0 is straight-chain alkyl or alkenyl having from 2 to 7 carbon atoms;
the medium essentially consists of compounds of the formulae I to VI and XIII;
the medium comprises further compounds, preferably selected from the following group consisting of the general formulae XIV to XVIII: 
in which R0 and X0 are as defined above. The 1,4-phenylene rings may additionally be substituted by CN, chlorine or fluorine. The 1,4-phenylene rings are preferably monosubstituted or polysubstituted by fluorine atoms.
The medium additionally comprises one, two, three or more, preferably two or three, compounds of the formulae 
in which xe2x80x9calkylxe2x80x9d and xe2x80x9calkyl*xe2x80x9d are as defined below. The proportion of the compounds of the formulae O1 and/or O2 in the mixtures according to the invention is preferably 5-10% by weight.
The medium preferably comprises 5-35% by weight of compound IVa.
The medium preferably comprises one, two or three compounds of the formula IVa in which X0 is F or OCF3.
The medium preferably comprises one or more compounds of the formulae IIa to IIg 
in which R0 is as defined above. In the compounds of the formulae IIa-IIg, R0 is preferably methyl, ethyl, n-propyl, n-butyl or n-pentyl.
The medium preferably comprises one or more compounds of the formulae 
in which R0 is as defined above.
The (I):II+III+IV+V+VI) ratio by weight is preferably from 1:10 to 10:1.
The medium essentially consists of compounds selected from the group consisting of the general formulae I to XIII.
The proportion of the compounds of the formula IVb and/or IVc in which X0 is fluorine and R0 is CH3, C2H5, n-C3H7, n-C4H9 or n-C5H11 in the mixture as a whole is from 2 to 20% by weight, in particular from 2 to 15% by weight.
The medium preferably comprises compounds of the formulae II to VI in which R0 is methyl.
The medium particularly preferably comprises compounds of the formulae 
The medium preferably comprises one, two or more, preferably one or two, dioxane compounds of the formulae 
The medium additionally comprises one, two or more bicyclic compounds of the formulae Z1 to Z6
in which R1a and R2a are each, independently of one another, H, CH3, C2H5 or n-C3H7. R0, alkyl and alkyl* are as defined in claim 3 or as defined below.
Of the said bicyclic compounds, particular preference is given to the compounds Z-1, Z-2, Z-5 and Z-6.
The medium additionally comprises one, two or more compounds having fused rings, of the formulae AN1to AN11: 
in which R0 is as defined above.
It has been found that even a relatively small proportion of compounds of the formulae I mixed with conventional liquid-crystal materials, but in particular with one or more compounds of the formulae II, III, IV, V and/or VI, results in a lowering of the threshold voltage and in low birefringence values, with broad nematic phases with low smectic-nematic transition temperatures being observed at the same time, improving the shelf life. Preference is given, in particular, to mixtures which, besides one or more compounds of the formulae I, comprise one or more compounds of the formula IV, in particular compounds of the formula IVa in which X0 is F or OCF3. The compounds of the formulae I to VI are colorless, stable and readily miscible with one another and with other liquid-crystalline materials.
The term xe2x80x9calkylxe2x80x9d or xe2x80x9calkyl*xe2x80x9d covers straight-chain and branched alkyl groups having 1-7 carbon atoms, in particular the straight-chain groups methyl, ethyl, propyl, butyl, pentyl, hexyl and heptyl. Groups having 1-5 carbon atoms are generally preferred.
The term xe2x80x9calkenylxe2x80x9d covers straight-chain and branched alkenyl groups having 2-7 carbon atoms, in particular the straight-chain groups. Preferred alkenyl groups are C2-C71E-alkenyl, C4-C73E-alkenyl, C5-C74-alkenyl, C6-C75-alkenyl and C7-6-alkenyl, in particular C2-C71E-alkenyl, C4-C73E-alkenyl and C5-C74-alkenyl. Examples of part preferred alkenyl groups are vinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groups having up to 5 carbon atoms are generally preferred.
The term xe2x80x9cfluoroalkylxe2x80x9d preferably covers straight-chain groups having a terminal fluorine, i.e. fluoromethyl, 2-fluoroethyl, 3-fluoropropyl, 4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and 7-fluoroheptyl. However, other positions of the fluorine are not excluded.
The term xe2x80x9coxaalkylxe2x80x9d preferably covers straight-chain radicals of the formula CnH2n+1xe2x80x94Oxe2x80x94(CH2)m, in which n and m are each, independently of one another, from 1 to 6. Preferably, n=1 and m is from 1 to 6.
Through a suitable choice of the meanings of R0 and X0, the addressing times, the threshold voltage, the steepness of the transmission characteristic lines, etc., can be modified in the desired manner. For example, 1E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxy radicals and the like generally result in shorter addressing times, improved nematic tendencies and a higher ratio of the elastic constants k33 (bend) and k11 (splay) compared with alkyl or alkoxy radicals. 4-alkenyl radicals, 3-alkenyl radicals and the like generally give lower threshold voltages and smaller values of k33/k11 compared with alkyl and alkoxy radicals.
A xe2x80x94CH2CH2xe2x80x94 group generally results in higher values of k33/k11 compared with a single covalent bond. Higher values of k33/k11 facilitate, for example, flatter transmission characteristic lines in TN cells with a 90xc2x0 twist (in order to achieve grey shades) and steeper transmission characteristic lines in STN, SBE and OMI cells (greater multiplexability), and vice versa.
The optimum mixing ratio of the compounds of the formulae I and II+m+IV+V+VI depends substantially on the desired properties, on the choice of the components of the formulae I, II, III, IV, V and/or VI, and on the choice of any other components that may be present. Suitable mixing ratios within the range given above can easily be determined from case to case.
The total amount of compounds of the formulae I to XIII in the mixtures according to the invention is not crucial. The mixtures can therefore comprise one or more further components for the purposes of optimization of various properties. However, the observed effect on the addressing times and the threshold voltage is generally greater, the higher the total concentration of compounds of the formulae I to XIII.
In a particularly preferred embodiment, the media according to the invention comprise compounds of the formulae II to VI (preferably II, III and/or IV, in particular IVa) in which X0 is F, OCF3, OCHF2, OCHxe2x95x90CF2, OCFxe2x95x90CF2 or OCF2xe2x80x94CF2H. A favorable synergistic effect with the compounds of the formulae I results in particularly advantageous properties. In particular, mixtures comprising compounds of the formula I and of the formula IVa are distinguished by their low threshold voltages.
The individual compounds of the formulae I to XVIII and their sub-formulae which can be used in the media according to the invention are either known or can be prepared analogously to the known compounds.
The construction of the MLC display according to the invention from polarizers, electrode base plates and surface-treated electrodes corresponds to the conventional construction for displays of this type. The term conventional construction is broadly drawn here and also covers all derivatives and modifications of the MLC display, in particular including matrix display elements based on poly-Si TFT or MIM.
A significant difference between the displays according to the invention and the conventional displays based on the twisted nematic cell consists, however, in the choice of the liquid-crystal parameters of the liquid-crystal layer.
The liquid-crystal mixtures which can be used in accordance with the invention are prepared in a manner conventional per se. In general, the desired amount of the components used in the lesser amount is dissolved in the components making up the principal constituent, advantageously at elevated temperature. It is also possible to mix solutions of the components in an organic solvent, for example in acetone, chloroform or methanol, and to remove the solvent again, for example by distillation, after thorough mixing.
The dielectrics may also comprise further additives known to the person skilled in the art and described in the literature. For example, 0-15% of pleochroic dyes or chiral dopants can be added.
C denotes a crystalline phase, S a smectic phase, SC a smectic C phase, N a nematic phase and I the isotropic phase.
V10 denotes the voltage for 10% transmission (viewing angle perpendicular to the plate surface). ton denotes the switch-on time and toff the switch-off time at an operating voltage corresponding to 2.0 times the value of V10. xcex94n denotes the optical anisotropy. xcex94xcex5 denotes the dielectric anisotropy (xcex94xcex5=xcex5∥xe2x88x92xcex5xe2x8axa5, where xcex5∥ denotes the dielectric constant parallel to the longitudinal molecular axes and xcex5xe2x8axa5 denotes the dielectric constant perpendicular thereto). The electro-optical data are measured in a TN cell at the 1st minimum (i.e. at a dxc2x7xcex94n value of 0.5 xcexcm) at 20xc2x0 C., unless expressly stated otherwise. The optical data are measured at 20xc2x0 C., unless expressly stated otherwise.
In the present application and in the examples below, the structures of the liquid-crystal compounds are indicated by means of acronyms, the transformation into chemical formulae taking place in accordance with Tables A and B below. All radicals CnH2n+1 and CmH2m+1 are straight-chain alkyl radicals having n and m carbon atoms respectively; n and m are integers and are preferably 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12. The coding in Table B is self-evident. In Table A, only the acronym for the parent structure is indicated. In individual cases, the acronym for the parent structure is followed, separated by a dash, by a code for the substituents R1*, R2*, L1* and L2*:
Preferred mixture components are given in Tables A and B.
Particular preference is given to liquid-crystalline mixtures which, besides the compounds of the formula I, comprise at least one, two, three or four compounds from Table B.
The entire disclosure of all applications, patents and publications, cited herein and of corresponding German patent application No. 102 23 061.7, filed May 24, 2002 is incorporated by reference herein.